The following U.S. Patent Applications by the same inventor, Elbert L. McKague, Jr. are being simultaneously filed and incorporated herein by reference: U.S. Patent Application entitled xe2x80x9cComposite Structural Panel with Undulated Bodyxe2x80x9d; U.S. Patent Application entitled xe2x80x9cApparatus And Method For Joining Dissimilar Materials To Form A Structural Support Memberxe2x80x9d; and by the same inventor together with inventors Ronald P. Schmidt and David T. Uhl, U.S. Patent Application entitled xe2x80x9cComposite Material Support Structures With Sinusoidal Webs And Method Of Fabricating Samexe2x80x9d.
1. Technical Field
The present invention relates in general to an improved material and method for enabling conformance to a tool surface that is incompatible with a planar starting configuration or an inextensible material, and in particular to an improved method for controlling and limiting the effects of properties damage that result from achieving conformance by creating fiber segments in an inextensible material. Still more particularly, the present invention relates to randomly distributing and arranging segment zones of fibers such that minimum and maximum fiber lengths are maintained between zones that encompass a vector parallel to the fiber axis.
2. Description of the Prior Art
Structural support spars or I-beams typically have an xe2x80x9cIxe2x80x9d shaped cross-section, having a web with a crossbar or flange on each end of the web. The web and flanges extend integrally down the length of the beam, but may vary in shape, thickness, etc. For example, a beam with a sinusoidal or sine wave-shaped web increases the apparent section thickness of the web. Beams with sinusoidal webs have been pursued through a variety of design and manufacturing approaches since these structures offer the potential of providing the best stiffness and strength-to-weight performance of any support structures.
Currently, all composite beams having undulated webs utilize composite materials with fibers that extend continuously from the web into the flanges. The fibers are simply turned to the desired angle (90 degrees in beams having flanges that are perpendicular to the web) prior to curing resin in the beam. In essence, two xe2x80x9cCxe2x80x9d shaped sections are created and then joined back-to-back by co-curing the web area and the flange portions to outer cap laminates that laterally unite the two C-sections.
Fabrication typically involves a great deal of hand working of the composite material into the undulated configuration. An especially labor-intensive step is required to properly turn the fibers at the corners where they transition from the curved to flat geometry between the web and flange portions of the beam. Many techniques have been used to create this configuration including lay-up and cure of previously impregnated (xe2x80x9cprepregxe2x80x9d) materials, and lay-up of dry fabric followed by resin transfer molding or other resin infusion methods. Because of the inherent manufacturing limitations of these techniques, fabric materials rather than unidirectional materials are usually required. However, simple, low-cost fabrication of composite structures with undulated shapes, such as beaded or corrugated stiffening elements, has not been possible because inextensible fibers cause bridging or wrinkling. This problem is often present even when laid layer by layer to the contour of the forming tool.
An early approach to making discontinuous, fiber prepreg involved stretch-breaking of collimated fibers by tensioning while passing over and around tensioning rollers. In this disclosure, the term xe2x80x9ccollimatedxe2x80x9d is used to describe continuous, inextensible fibers which align in exactly the same direction. Typically, this solution produced long, discontinuous fibers that possessed individual fiber segmentation qualities as opposed to bundle segmentation qualities. Although the resulting fibrous material can be impregnated with powdered thermoplastic resin employing electrostatic attraction methods, the material cannot be impregnated with hot melt thermosetting resins without having numerous fibers peel and entangle as they become trapped by the associated machinery. These problems caused serious manufacturing and quality problems. Furthermore, the resulting long fibers span the convex peaks separating convex areas of many parts.
Slits of precisely controlled length are cut in a layer of material having collimated tows of fibers. Each slit begins at a defined coordinate point and proceeds diagonally at an acute angle relative to the axial direction of the fibers. Subsequent slits are initiated at coordinate points relative to the starting coordinate point of the previous slit. The region of fibers located above and below each slit are referred to as upper and lower fiber bands, respectively. The lower bands are substantially aligned in a zonal band that runs perpendicular to the lengths of the fibers. The location of the beginning coordinate point for each slit defining a zonal band is defined by adding a random length dimension to a previous starting coordinate point.
The maximum length of the cut fibers may be determined in accordance with the intended application and process. For example, in the case of a corrugated shape that is made by forming a flat laminate into the final corrugated shape in one step, the maximum fiber length is limited to a dimension somewhat less than the period of the corrugations. Such a limitation prevents the fiber ends from being pinned against adjacent peaks of the tool. The minimum length of the fibers is selected to assure maximum load-carrying capacity of the finished composite component. The length must be great enough to transfer a shear load equal to the inherent tensile capacity of the fiber along the fiber surface between itself and an adjacent fiber coupled to it by a resin matrix.
The foregoing and other objects and advantages of the present invention will be apparent to those skilled in the art, in view of the following detailed description of the preferred embodiment of the present invention, taken in conjunction with the appended claims and the accompanying drawings.